The present invention relates to a microwave delay line for a travelling-wave tube incorporating an electrical conductor, whose geometry varies only along the axis of the tube.
As is known, a travelling-wave tube is constituted by the association of a long thin electron beam with a non-resonant delay line having a periodic structure. The electrons of the beam supply energy to the microwave travelling along the line when certain conditions of synchronism exist between the wave and the beam. The delay line is generally constituted by a helix, or a circuit derived from a helix. The electrons are transmitted in accordance with the helix axis, which is also the tube axis. Circuits derived from helix, include the multiple conductor helix having two intertwined leads, a counter-helix or its topological equivalents, or the ring and bar or ring and loop circuit, or a microwave structure whose mechanical connection to the tube envelope is provided by quarter wavelength metal supports. For simplicity in the detailed description, the delay lines hereinafter will be a simple helix.
In the prior art, the electrical efficiency of a travelling-wave tube is an increasing function of the coupling impedance between the electron beam and the delay line. This has led to a maximum desired coupling impedance. It is obtained by a width of conductor wire constituting a simple helix approximately equal to half the helix pitch. The width being constant along the tube axis.